The present invention relates to an improved printed circuit manufacturing process. More particularly, it relates to a process for manufacturing a printed circuit incorporating tin-nickel plating.
The electronics industry has expanded dramatically over the last several decades, in terms of types of product available and their functional capabilities. Printed circuit technology has been, and continues to be, a vital catalyst for this expansion. Each year, new electronic products are introduced, and previously-existing ones are improved. Virtually every one of these electronic products, including computers, telephones, calculators, etc., includes a printed circuit.
The requirements placed on printed circuit capabilities have increased greatly in conjunction with the constant demand for improved electronics technology. Multilayer printed circuits having literally thousands of circuitry traces and connectors are now commonplace. Additionally, as electronic products continue to become smaller in size, the printed circuit associated with the individual product must also decrease in size, resulting in tighter manufacturing tolerances. Even further, printed circuit manufacturers are now being asked to produce larger volumes of identical printed circuits within shorter turnaround times. To accommodate these requirements, the manufacturing techniques associated with printed circuits have evolved.
In addition to providing dense, high performance printed circuits, today's printed circuit fabrication industry must remain cognizant of environmental concerns. It is not enough to simply manufacture a printed circuit having increased capabilities; the manufacturing process itself must be environmentally safe. For example, intricate waste treatment systems have been developed in an attempt to minimize the environmental impact of hazardous waste inherently associated with printed circuit fabrication.
Finally, to remain competitive, printed circuit manufacturers must keep fabrication costs as low as possible. This is an extremely difficult task, given that printed circuit manufacturing entails a number of independent processing steps, as described in greater detail below. Each individual process includes its own associated costs, such as, for example, requiring the specialized waste treatment system mentioned above.
Generally speaking, the above stated goals of increased capabilities along with reduced environmental hazards and costs are inherently opposed. Standard manufacturing techniques require the use of hazardous chemicals, such as acids and oxidizers. In addition, lead-based solder has heretofore been an integral component of the printed circuit manufacturing process. For example, a typically employed printed circuit manufacturing process begins with a non-conductive substrate covered with a copper foil layer. A resist (or plating resist) is applied to the copper foil. A film (or artwork) containing an image of a desired circuitry pattern is associated with the resist-covered copper surface and exposed to a light source. Subsequently, the board is passed through a "developing" process to selectively remove some of the resist from the board. More particularly, following developing, resist is removed from the desired circuitry pattern, but encompasses the remainder of the board surface. In other words, the desired circuitry pattern no longer has any resist, such that the copper foil surface is exposed. The remainder of the copper foil, however, remains coated with the resist material.
The board is processed through an electroplating process by which additional copper is electroplated to the exposed circuitry pattern. Any areas covered by resist are not plated with copper. Thus, the exposed circuitry pattern is "plated up" with additional copper.
Tin or tin-lead (or solder) is electroplated over the previously-plated copper. Once again, the resist prevents any of the tin or tin-lead material from plating to any board surface covered with the resist. Following the tin or tin-lead electroplating process, then, the desired circuitry pattern is defined by the previously-plated copper and a layer of tin or tin-lead. The remainder of the board otherwise covered with resist does not have additional material plated to the copper foil surface. In this regard, the tin or tin-lead serves as a barrier to prevent subsequent etching of the desired circuitry pattern and acts as a protective coating on the plated copper to prevent oxidation.
The board is then processed through a "stripping" procedure during which the resist is stripped away (or removed) from the board surface. With the resist removed, the copper foil previously covered by the resist is now exposed.
The board is then subjected to an "etching" process. The etching process acts to etch or remove copper from the surface of the board not otherwise protected by the tin or tin-lead material. Any copper foil not covered is entirely etched from the substrate surface. As a result, the desired circuitry pattern, defined by copper foil, electroplated copper, and a tin or tin-lead layer, remains adhered to the laminant surface. All other copper, however, is now removed. Where tin-lead (or solder) is used as the etch resistant, the tin-lead is then cleaned in a conditioner and a flux. Following this processing, the solder is "reflowed" using infrared heat or "hot oil". The reflowed solder forms a protective alloy over the entire copper trace.
Alternatively, with a fabrication method referred to as Solder Mask Over Bare Copper (SMOBC), tin (as opposed to tin-lead) is used as the etch resistant. With this technique, following etching, the tin material is stripped (or removed) from the circuitry pattern. As a result, the desired circuitry pattern is in the form of plated copper traces adhered to the substrate.
A solder mask is then applied to the board as a protective coating for the various copper traces. In particular, the solder mask is coated on the board in a predetermined pattern, leaving selected areas of the circuitry pattern unprotected (or "bare"). Following application of the solder mask, the board is treated with a substance, most commonly solder, which adheres to any areas not covered by the solder mask. One such solder application technique is referred to as Hot Air Leveling (HAL) in which the board is immersed in a molten solder bath of 63% tin and 37% lead. Excess solder is removed from the board and any previously drilled holes by forced air. An additional goal of the HAL process is to "level" the solder that remains adhered to selected portions of the circuitry pattern. In theory, the leveling process results in a flat surface for subsequent surface mount technology (SMT) applications. In practice, however, HAL cannot produce a perfectly planar surface. HAL oftentimes results in a "crown"-like surface, hindering proper chip SMT placement. Even with a relatively flat HAL-produced surface, chip SMT application requires a wave solder step to secure the chip or other component to the board surface. Wave soldering, while widely accepted, may result in undesired chip migrations as the solder liquifies.
It should be noted that recently, efforts have been made to replace the solder-hot air leveling process with the use of a clear organic polymer, an immersion tin, or an immersion/electroless palladium material placed over the copper. These processes have not been proven to eliminate the above-described problems, and instead add an additional manufacturing step (with additional costs).
A subsequent procedure sometimes employed in printed circuit fabrication is plating nickel and gold to select areas of the circuitry pattern, such as surface mount pads and/or tabs. With this application, solder or tin is first selectively stripped and the underlying copper cleaned. Nickel is then electroplated to desired areas of the printed circuits followed by gold electroplating. Electroplating of nickel is required to prevent "migration" of copper into the electroplated gold. Basically, the layer of electroplated nickel acts as a metallic barrier, thereby preventing copper migration. The migration of copper into gold is undesirable, as copper reduces the anti-corrosive properties of gold, which is essential to the integrity of printed circuit contacts, requiring exposed conductive leads, such as keypads and contact tabs.
The printed circuit fabrication process has been described above in very basic terms. It should be clear, however, that printed circuit manufacturing is quite complex. Boards are subjected to a wide variety of chemical solutions and mechanical processing, many resulting in formation of waste materials. Additionally, each process step inherently increases overall production costs, along with presenting a greater opportunity for error. Finally, several of the processing steps subject the board to possible long-term damage. For example, HAL subjects a board to extreme thermal shock.
Printed circuits continue to be an integral part of the evolution of the electronics industry. To this end, any technique able to maintain or even improve board integrity and performance while eliminating process steps, costs and hazardous waste is highly desirable. Therefore, a substantial need exists for a printed circuit manufacturing process with reduced steps, costs and waste.